Guy grew up at Loxley with Robin and Scarlett. His parents, who were previously low-ranked nobility, lost their noble status due to a dispute/scandal with Robin's father, William of Loxley. Due to which, they ended up indebted to the Loxley family and started working at the Huntington estate as servants.
While in early childhood Guy and Robin were close friends, over time their social difference, Guy's position as a servant to Robins family, as well as some other issues, especially with William (which I won't talk about just yet) became more and more noticeable — especially to Guy — causing him to slowly start resenting and envying Robin.
And one day, the resentment that had been building up in Guy came to a boiling point and they ended up having a big fall-out shortly before Robin, Scarlett and Mathilda left for Nottingham while Guy stayed behind at Huntington.
Eventually (after some time of Robin starting to live in Sherwood and accidentally becoming famous) word about Robin Hood, his friends and all the mischief they get up to spreads across England and, sooner or later, Guy also gets wind of it.
Guy rather quickly decided to go after him, quietly leaving Loxley one night without telling anyone (which had him assumed dead by most people there) and made his way to Nottingham, seeking an audience with Prince John.
On his arrival at the castle, he pledged his allegiance to John with the promise to put an end to the "difficulties" Robin had been causing him.
He gets a position as the Sheriff's apprentice and, over time, is able to prove his usefulness — the fact that he previously spent most of his childhood by Robins' side, training with him, etc. making it easier for him to keep up with and even predict some of Robins' plans — and gain Johns trust (after which they gradually continued growing closer).
On his arrival at the castle, he pledged his allegiance to John with the promise to put an end to the "difficulties" Robin has been causing him.
He gets a position as the Sheriffs apprentice and over time is able to prove his usefullness — the fact that he previously spent most of his childhood by Robins side, training with him, etc. making it easier for him to keep up with and even predict some of Robins plans — and gain Johns trust (after which they gradually continued growing closer).
And shenanigans ensue from here on out
(& some more drawings of him at the end)